The present invention relates to a method and apparatus for drying a sheet of veneer and contemplates providing a rationalized drying technique which minimizes or practically eliminates splitting in a veneer sheet which conventionally resulted from such treatment.
Traditionally, a process for producing plywood includes a step of drying veneer sheets with a drier. When dried, however, veneer sheets crack concentrically at irregularly distributed spots and become very poor in quality even though they may have been of favorable quality before the drying step. Cracked parts of veneer sheets must be cut off and wasted with the resultant decrease in the yield and this critically reduces the output from veneer sheets.
To cope with this problem, there has been proposed and put to practical use, though in a minor part of the industry, a process by which a veneer sheet is first formed with numerous short slits to be tenderized and then allowed to go on the drying step (referred to as the "Y process" hereinafter). Compared with the process without tenderizing (referred to as the "X process" hereinafter), the Y process somewhat decreases the tendency of veneer sheets to split concentrically during drying and has given an acceptable result so far. However, such a technique is not fully acceptable in that it still fails to entirely eliminate splits attributable to the drying step.
It is apparent that the splits in a veneer sheet caused by drying and commonly observed in the conventional processes result from contraction of the veneer sheet when subjected to such treatment. Where the conveying members within a drier comprises upper and lower nets as in a net drier for example, they obstruct the contraction of a veneer sheet which should be allowed gradually in the course of the drying step and allow stresses to develop within the veneer sheet. The veneer sheet breaks or cracks in locations where the stresses are the most concentrated. With this in view, the Y process employing tenderizing process is designed to avoid cracks by decentralizing the stresses in the veneer sheet by means of numerous short slits and thereby reduce the tendency of splitting as a whole. Extended studies which we made to determine the benefit of tenderized veneer sheets showed that the range of elasticity in which a veneer sheet stretches without splitting when pulled in a direction perpendicular to its grain is larger in tenderized sheets than in non-tenderized sheets.